Conventional heating systems regulate the temperature of the heating element using two separate devices. A temperature sensor measures the temperature of the heating element, sending a signal to a temperature controller. The temperature controller evaluates the signal to determine when to supply power to the heating element, keeping the temperature within a predetermined range. Compensation wires connect the temperature signal to the temperature controller.
In a conventional heating system, one temperature sensor with a corresponding compensation wire is needed for each heating element. If the system contains several heating elements, it will require several temperature sensors and compensation wires. While the conventional system sufficiently regulates the temperature of the heating elements, it is quite bulky. As a result, the number of possible applications for the system is limited.
The actual temperature of a heating element is not uniform throughout the device. The temperature sensor detects the temperature of that portion of the element to which it is attached. The measured temperature will vary with the positioning of the temperature sensors. The measured temperature also deviates from the actual temperature even at that single location because of a propagation delay in the interface between gas, liquid, and solid. Some situations require precise temperature measurements to closely regulate a heating system.
U.S. Pat. No. 3,789,190 issued to D. Orosy et al on Jan. 29, 1974 discloses an electrical heating device which incorporates heating elements that also function as temperature sensors. The Orosy heating element uses materials in which the resistance changes linearly with respect to a change in temperature. One heating element serves as the fourth leg of a Wheatstone bridge. A change in temperature causes a change in resistance which results in a bridge imbalance. The system uses the bridge imbalance to regulate the power supply to the heating element. This regulation is accomplished by turning the power supply to the heating elements on and off for variable periods of time dependent upon the temperature of the heating element. The Orosy temperature control system offers more precision when regulating a single heating element than the conventional heating system.
Another prior art heating systems controls the temperature of the heating elements by connecting or disconnecting the power supply once the measured temperature leaves the desired range. When the temperature drops below the lower limit, the power supply is connected to the heating element, causing the temperature to increase. When the measured temperature reaches the upper limit, the power supply is disconnected and the temperature gradually decreases. As a result, the temperature oscillates back and forth between the upper and lower limit, spending little time at a single desired temperature. A heating system which regulates a heating element by adjusting the power supply level would have better control over the actual temperature of the element. Instead of supplying full power over variable time periods as in the prior art, the system could supply power in increments, thus preventing the extreme oscillation of the conventional systems.
Certain conditions require a heating system which regulates several heating elements with a central control unit. In such multiple element heating systems, the use of heating elements which function as temperature sensors is less bulky and more accurate than the conventional system. Such a heating system which controls several heating elements with a central control unit is even more accurate since each element can be regulated with respect to the others. A central control unit eliminates the need for a controller for each heating element, reducing the number of parts needed for the system. This further reduces the overall bulk of the heating system. A central control also improves the reliability of the system.